I would like see the ENY Cap removed in favor of a sliding perk scale. That means instead of an aircraft being unusable because of ENY, it simply would have a perk cost relative to its ENY. The equation I would recommend is P=M(1/E)(C-E+1)+I, when C≥E. So to break this down:
P= the ENY perk cost
M= Some constant
E= A plane's ENY
C= The ENY cap
I= Any initial perk cost
Lets use the P-51D as an example:
So lets say the ENY cap is at 5 and Hitech wanted M to be set at 100, so plug everything in you get P=100(1/5)(5-5+1)+0=20. So this means when the ENY cap is at 5, the P-51D will have a perk cost of 20.
Now lets say the ENY cap rises to 7, again P=100(1/5)(7-5+1)+0=60. So as the ENY cap rises to 7, the 51's cost grows to 60.
This all changes with a different M value, which can be tuned at Hitech's discretion. Lets say the ENY is at 10, the M=10, and we have the Tempest with a base cost of 50, so P=10(1/5)(10-5+1)+50=62. So with M at 10 and ENY at 10, the Tempest would cost 62 perks (170 perks when M=100).
This still allows anyone to fly anything they want to, just make them prohibitively expensive....